A two-stage gas-cleaning electrostatic precipitator usually consists of an ionizing stage, a collecting stage and a fan for causing the particle laden gas to pass through the ionizing stage and then through the collecting stage. The particles to be removed from the gas are ionized or given a charge when the gas passes through the ionizing stage. The charged particles pass into the collecting stage where they are precipitated. The precipitation occurs because there is a voltage gradient in the spaces between the plates of the collecting stage which acts on the charged particles, moving them to the plates and thus out of the gas stream. Once the particles are precipitated in the collecting stage they must be removed from the plates so that more particles can be collected.
U.S. Pat. No. 2,911,060 describes a cleaning system for continuously removing particles from the collecting surface of a large single-stage electrostatic precipitator. The system uses a hood which draws gas from the single-stage precipitator compartment. The cleaning air, drawn through the hood passes to a hopper where the gas velocity is so low that the removed dust can settle. An induced draft fan draws air from the hopper and either returns it to the inlet of the precipitator or passes it through other undescribed cleaning apparatus before exhausting it to the atmosphere.
U.S. Pat. No. 2,701,622 also shows a system for cleaning a single-stage electrostatic precipitator which rotates the precipitator passed the cleaning air duct. Collected dust is blown from the precipitator by the cleaning air and is collected in a cyclone type after-collector. The patent teaches that the cleaning air is maintained at a high temperature and is recirculated from the after-collector through the cleaning air duct to the precipitator.
Both of these patents are directed to single-stage precipitators which cannot be used for ventilating air because of ozone generation and which are typically used to collect high dust loadings. Moreover, the above patents appear to use relatively low plate cleaning air velocities, on the order of 3500 ft/min, although no numbers are actually given, which would be ineffective in cleaning the plates of a two-stage electrostatic precipitation.
As compared to a single-stage electrostatic precipitator, a two-stage electrostatic precipitator is a much smaller device, uses less power, and can be made to generate only a minute amount of ozone so that it can be used to clean ventilating air. One serious drawback, however, is that the collected particles cannot be held onto the collecting plates electrically but are held on only by adhesion. With dry particulates, the adhesion property varies dramatically depending upon the composition of the particulates. This has seriously limited the field of application of two-stage electrostatic precipitators.
Another drawback is the low dust holding capacity of two-stage precipitators. To a first approximation, a given cleaning capacity measured in cubic feet per minute (CFM) requires a given particle or dust collecting area. Reducing the spacing between electrodes as described in U.S. Pat. No. 2,129,783 not only reduces the size of a two-stage precipitator, but also reduces the dust holding capacity. Consequently, a two-stage electrostatic precipitator, typically, has been used only for relatively low particle loadings except for the case of oil droplets where the collected liquid can continuously drain from the collecting electrodes.
Typically, two-stage electrostatic precipitators are operated with a gas velocity at the face of the collecting section of 300-400 ft/min. At this gas velocity and with high particle loadings the collecting section will require frequent cleaning. Normally this is done by shutting down the precipitator and removing the collecting section for cleaning with water. But washing requires a period for drying before voltage can be reapplied so washing is not an acceptable cleaning method for maintaining high efficiency. A preferable cleaning mechanism would be one which can function effectively during the operation of the precipitator without shutting it down.
U.S. Pat. No. 2,672,947 shows a cleaning system in a two-stage precipitator which does not require the shutting down of the gas flow. However, this system requires that the collecting sections have a total cross sectional area for gas flow which is greater than that of the gas flow to be cleaned. This is because each of the collecting sections in turn is removed from the gas flow during cleaning. Additionally, this system requires a special device for first reducing and then eliminating the voltage in a collecting section during cleaning. It would be desirable to have a simpler cleaning device which did not require the removal of collecting sections during cleaning or changes in precipitator voltage.
There is a need, therefore, for a two-stage electrostatic precipitator capable of handling high particle loadings which includes a cleaning device which uses a small flow of cleaning gas at a very high velocity to effectively remove the dust from the dust collecting plates and which does not require the removal of collecting sections or the shutting down of the precipitator during cleaning.